Injection moulding of large plastic components

ABSTRACT

A system for injection moulding of large plastics components in a single shot comprises an extruder and an accumulator fed from the extruder The accumulator leads into the mould ( 14 ) via a mould valve ( 12 ) opive, when closed, to close the mould cavity substantially at the mould surface and also to close an outlet ( 20   a ) from the accumulator. The valve passage of the mould valve is of relatively large cross-section, typically several centimetres or more which facilitates injection at relatively low pressures and also facilitates passage of stones and other particulate matter which may be present if the extrudate is composed of post-consumer recycled plastics.

The present invention relates to the manufacture of moulded plasticsproducts and more particularly to the manufacture of plastics productsof substantial size and weight. More specifically, but not exclusively,the invention relates to the manufacture of plastics products frompost-consumer recycled plastics waste.

Conventional plastics injection moulding techniques traditionallyinvolve the use of high injection pressures. The mould must be capableof withstanding the injection pressures involved and as a result whenarticles of relatively large size are being produced the cost of theinjection equipment and mould is very substantial.

Traditional injection moulding equipment which relies on the use ofrelatively narrow runners leading into the mould requires the use ofvirgin plastic or recycled plastic that is substantially free of foreignmatter likely to interfere with the injection process. Post-consumerrecycled plastic is likely to contain particles of metal glass or stonewhich for most traditional injection moulding methods must be removedprior to re-use of the recycled plastics. The processing required toensure that the recycled plastics is substantially free of foreignmatter further adds to the cost of processing which may make the cost ofusing recycled plastics not commercially worthwhile.

The present invention seeks to provide a method and apparatus forrelatively low pressure injection moulding of large plastics articles inone shot possibly using post-consumer recycled plastics which may stillcontain particles of foreign matter.

According to the present invention, there is provided a method ofinjection moulding a plastics article under relatively low injectionpressures, comprising feeding plastics material from an extruder into anaccumulator, and displacing the extrudate from the accumulator into amould via a valve leading into the mould cavity, the valve having avalve piston which at the end of injection closes the mould cavitysubstantially at the mould surface.

In a preferred embodiment of the invention, during injection ofextrudate from the accumulator into the mould cavity the extrudercontinues in operation to provide further extrudate for use in the sameinjection cycle.

The extruder may be fed with plastics material composed at leastpartially of post-consumer recycled plastics, the valve passage being ofa size such that it will not be blocked by stones or other particulateforeign matter which might be present within the material. Therelatively large cross-sectional area of the valve passage also providesrelatively low pressure, but high volumetric flow, of injection into themould cavity. Advantageously, filling of the mould with the requiredamount of plastics material is determined by sensing of pressure withinthe mould cavity.

According to another aspect of the invention, there is provided aninjection moulding system for single shot injection moulding of largeplastics components, comprising an extruder, and an accumulator fed fromthe extruder, the accumulator leading into the mould via a mould valveoperative when closed to close the mould cavity substantially at themould surface and also to close an outlet from the accumulator.

In a preferred embodiment of the invention, the accumulator comprises anaccumulator cylinder having a piston operative to inject extrudate intothe mould, and the mould valve comprises a valve member in the form of apiston within a tubular valve passage terminating in an injection nozzlelying within the mould cavity, the valve piston having an end facewhich, in the closed condition of the valve, lies substantially at theoutlet end of the injection nozzle and seats within the nozzle.Advantageously, an outlet from the accumulator branches into the valvepassage, and, in the closed condition of the mould valve, a side wall ofthe piston closes the outlet at the position at which it branches intothe valve passage.

Still further according to the invention, there is provided a system forinjection moulding relatively large plastics articles, comprising anextruder, an accumulator coupled to an outlet from the extruder, theaccumulator leading into a mould via a mould valve having a passage ofrelatively large cross-sectional size whereby to effect injection atrelatively low injection pressures, and means for sensing a mould-fullcondition to effect closure of the mould valve by sensing the pressureof injected material at one or more positions within the mould cavity.

An embodiment of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings, in which:

FIG. 1 shows schematically an injection moulding system in accordancewith a preferred embodiment of the invention for one shot injectionmoulding of large plastics articles; and

FIG. 2 shows schematically and to an enlarged scale a mould valve withinthe system of FIG. 1 and illustrating the relationship between the mouldvalve and the mould cavity in a closed condition of the valve.

The system shown in the accompanying drawings comprises an extruder 2fed with raw plastics material via a suitable inlet device 4 whichdelivers a specific volume of raw material to the extruder 2 in order tokeep the extruder loaded with plastics material at all times duringoperation of the extruder. The incoming plastics material may consist ofpost-consumer recycled plastics material shredded and comminuted in sizeand possibly mixed with recycled raw plastics material according torequirements. The extrudate from the extruder 2 passes via a heatedtransfer pipe 6 to an accumulator 8 which is preferably of hydraulictype; the transfer pipe 6 includes a pressure relief valve 10 loaded byan air cylinder. The accumulator 8 in its preferred form consists of alarge capacity cylinder with a hydraulically-powered piston. Betweeninjection shots, the cylinder is filled with extrudate from the extruder2 and when the accumulator cylinder is filled the accumulator piston isactuated to inject extrudate via a mould valve 12 into the mould cavity14. During this process the extruder 2 continues to operate whereby themould cavity 14 is filled with extrudate previously stored in theaccumulator 8 and also continually fed from the extruder 2 via theaccumulator 8 during injection.

As shown in FIG. 2, the mould valve 12 comprises a tube 16 defining avalve passage in which is mounted a hydraulically-actuated piston 18.The tube 16 extends through the wall 14 a of the mould into the mouldcavity 14. A feed line 20 from the accumulator 8 branches into the tube16 of the mould valve 12. In the open condition of the mould valve 12,the valve piston 18 is retracted to a position upstream of the branchinlet 20 a from the feed line 20 to enable free passage of the extrudateinto the mould cavity 14 via the tube 16 and via a heated nozzle 22 atthe outlet end of the tube 16. In the closed condition of the mouldvalve 12 (as shown in FIG. 2) the piston 18 not only closes the branchinlet 20 a from the accumulator feed line 20, but also extends via tube16 into the mould cavity 14, the end of the valve piston 18 seatingwithin the nozzle 22, whereby the nozzle 22 defines the seat for thevalve piston 18 in its closed condition so that the end of the piston 8shuts off the mould cavity substantially at the mould surface; i.e. thesurface of the article being moulded. As a result, the moulding is notformed with any sprues which require post-forming finishing operations.

The tube 16 defining the valve passage and the injection nozzle 22through which extrudate is injected into the mould are of relativelylarge diameter, typically of several centimetres or more up to about 10centimetres diameter depending on the size of the mould, and are suchthat stones or similarly large particles of foreign matter possibly upto a size of 2-3 centimetres which might remain after shredding andcomminution of incoming post-consumer plastics material, will notinterfere with the injection process. The large cross-section of theinjection system also facilitates a high volumetric flow rate at lowerinjection pressures than is conventional.

The system is particularly suitable for one shot moulding of plasticscomponents of many tens of kilograms in weight, for example componentsof up to about 130 kilograms in weight. By way of illustrative exampleonly, the system can be used for one shot moulding of load-carryingpallets. A typical injection pressure using the system of the preferredembodiment will be of the order of 400 pounds per square inch which ismany times less than that used in conventional injection mouldingprocesses.

Conventional injection moulding processes which operate using highinjection pressures usually inject into the mould on a time basis orvolumetric basis. Although either basis is likely to provide consistentresults when operating at high injection pressures, it is unlikely to doso at the much lower injection pressures which take place using thesystem of the preferred embodiment of the present invention,particularly when using post-consumer recycled plastics as the extrudateitself will not always be consistent and may also be subject to someinternal passing arising from foreign matter within the mix. To overcomethis difficulty, one or more pressure sensors are incorporated atappropriate positions within the mould to sense build-up of internalpressure within the mould during injection. Complete filling of themould will be determined by sensing of the required internal pressurewithin the mould cavity by the or each pressure sensor within the mouldand at that point the mould valve 12 will be closed with the piston 18of the valve closing directly onto the mould surface as previouslydiscussed; injection from the accumulator 8 will also cease and theaccumulator piston will be retracted to permit re-charging of theaccumulator with extrudate in preparation for the next mould cycle. Thenumber of pressure sensors within the mould and the positioning of thepressure sensors will be dependent on the volume of the mould and alsothe complexity of the mould shape.

As the system is operating at comparatively low injection pressures, theoverall mould structure can be built to a lesser strength than thatwhich would be needed if substantially higher injection pressures werebeing used and this, in turn, will lead to a significantly reduced mouldcost. The relatively large volumetric injection flow rate at reducedpressures also results in absence of high back pressure onto theextruder whereby the extruder can operate at substantially constantspeed throughout its cycle and without undergoing substantial stressvariations throughout its cycle thus resulting in increased working lifefor the extruder.

It is to be understood that although the system described isparticularly suitable for use with plastics composed at least partiallyof post-consumer recycled plastics it can also be used with raw plasticsmaterial.

Throughout this specification and claims which follow, unless thecontext requires otherwise, the word “comprise”, and variations such as“comprises” or “comprising”, will be understood to imply the inclusionof a stated integer or group of integers or steps but not the exclusionof any other integer or group of integers.

The embodiment has been described by way of example only andmodifications are possible within the scope of the invention.

What is claimed is:
 1. A method of injection moulding a plasticsarticle, comprising feeding plastics material from an extruder into anaccumulator, and displacing the extrudate from the accumulator into amould via a valve leading into the mould cavity, the valve having avalve piston which, at the end of injection, closes the mould cavitysubstantially at the mould surface of the article whereby the mouldingis formed without the presence of a sprue at the site of injection, andfilling of the mould with the required amount of plastics material isdetermined during moulding by sensing pressure of the plastics materialwithin the mould cavity, the valve being closed in response to amould-full condition as determined by the same sensing pressure.
 2. Amethod according to claim 1, wherein during injection of extrudate fromthe accumulator into the mould cavity the extruder continues inoperation to provide further extrudate for use in the same injectioncycle.
 3. A method according to claim 2, wherein the injection pressureis of the order of several hundreds of pounds per square inch.
 4. Amethod according to claim 1, wherein the extruder is fed with plasticsmaterial composed at least partially of post-consumer recycled plastics,the valve passage being of a size such that it will not be blocked bystones or other particulate foreign matter.
 5. A method according toclaim 1, wherein the valve piston also closes an outlet of theaccumulator at the end of injection.
 6. A method of injection moulding aplastics article, comprising feeding plastics material from an extruderinto an accumulator, and displacing the extrudate from the accumulatorinto a mould via a valve leading into the mould cavity, the valve havinga valve passage with a valve piston which at the end of injection closesthe mould cavity substantially at the mould surface of the articlewhereby the moulding is formed without the presence of a sprue at thesite of injection, the valve passage and piston being of a constantdiameter of at least several centimeters, and filling of the mould withthe required amount of plastics material is determined during mouldingby sensing pressure of the plastics material within the mould cavity,the valve being closed in response to a mould-full condition asdetermined by the same sensing pressure.
 7. An injection moulding systemfor single shot injection moulding of plastics components, comprising anextruder, an accumulator fed from the extruder, the accumulator leadinginto the mould via a mould valve operative, when closed, to close themould cavity substantially at the mould surface whereby the moulding isformed without the presence of a sprue at the site of injection and alsoto close an outlet from the accumulator, and means for determining thata required amount of plastics material has been fed into the mould bysensing the pressure of injected material at one or more positionswithin the mould cavity, the valve being closed in response to amould-full condition as determined by said pressure sensing.
 8. A systemaccording to claim 7, wherein the accumulator comprises an accumulatorcylinder having a piston operative to inject extrudate into the mould,and the mould valve comprises a valve member in the form of a pistonwithin a tubular valve passage terminating in an injection nozzle lyingwithin the mould cavity, the valve piston having an end face which, inthe closed condition of the valve, lies substantially at the outlet endof the injection nozzle and seats within the nozzle.
 9. A systemaccording to claim 8, wherein the outlet from the accumulator branchesinto the valve passage, and, in the closed condition of the mould valve,a side wall of the valve piston closes the accumulator outlet at theposition at which it branches into the valve passage.
 10. A systemaccording to claim 7, wherein the valve passage and accumulator outletare of a size such that they will not be blocked by stones or otherparticulate matter in the extrudate when at least partially composed ofpost-consumer recycled plastics.
 11. A system for injection mouldingplastics articles, comprising an extruder, an accumulator coupled to anoutlet from the extruder, the accumulator leading into a mould via amould valve having a passage of a cross-sectional size such that it willnot be blocked by stones or other particulate foreign matter in theextrudate when at least partially composed of post-consumer recycledplastics, and means for determining that a required amount of plasticsmaterial has been fed into the mould by sensing the pressure of injectedmaterial at one or more positions within the mould cavity, the valvebeing oDerative to close the mould cavity substantially at the mouldsurface of the article whereby the moulding is formed without thepresence of a sprue at the site of injection in response to a mould-fullcondition as determined by said pressure sensing.
 12. A system accordingto claim 11, wherein the injection pressure is of the order of severalhundreds of pounds per square inch.
 13. A system according to claim 12,wherein the passage has a diameter of at least several centimeters.